This invention concerns systems and methods for encoding linear positioning information in amplitude-based servopositioning systems suitable for linear data recording on a medium such as magnetic tape.
Modem data storage systems use servopositioning (or xe2x80x9cservoxe2x80x9d) systems to guide their recording and playback components with respect to a recording medium, and thus enable high track density, which increases data storage capacity. Errors in the ability to follow the servopositioning signals on the medium can cause unacceptable reductions in storage capacity, recording/playback rates, and other parameters that are important to consumers (and thus to system manufacturers).
One type of servo patterns or formats for linear magnetic tape recording systems employs so-called time-based servo techniques, examples of which are disclosed in U.S. Pat. Nos. 5,689,384; 5,930,065; and 6,021,013 (all of which are incorporated by reference in their entireties). Commercial magnetic tape drives such as the IBM model 3570 and drives known under the names xe2x80x9cUltriumxe2x80x9d and xe2x80x9cAccelisxe2x80x9d as described by the Linear Tape Open consortium, use time-based servopositioning systems. In such systems, as taught in the patents noted above, one may encode linear position information by a type of common mode timing modulation.
Other types of servo schemes having encoded linear position information include that taught in U.S. Pat. No. 6,134,070, in which encoding is accomplished by modulating the size of the xe2x80x9cwindowxe2x80x9d portions of a servopositioning pattern. Another approach is disclosed in U.S. Pat. No. 5,920,439, in which a carrier burst of increased frequency represents a digital value. Yet another approach, disclosed in U.S. Pat. No. 5,973,869, synchronizes position error signal and linear position signals by using a jump in carrier frequency.
The invention is a servopositioning method and system for a data recording system employing a linear data recording medium, preferably magnetic recording tape. An amplitude-based servo signal format is written or recorded on the medium. The format comprises first, second, third, and fourth sections, each section having a respective amplitude, and each written over a respective period. The first and second sections together comprise a first interval, and the third and fourth sections together comprise a second interval. In each interval, the amplitudes of the sections are different from each other, and thus the system further comprises circuitry responsive to the signal amplitudes of the intervals for determining a position error signal (PES), i.e., a signal which measures the distance the servo head is away from the desired track center.
In one embodiment, the mean amplitude of the first and second intervals are different from each other, but the PES, because it is normalized, is not sensitive to this difference in mean amplitudes. That is, PES is proportional to the ratio of the amplitude of the second section to that of the first section, as well as to the ratio of the amplitude of the fourth section to that of the third section. The system further comprises circuitry responsive to signal amplitude for decoding linear position information encoded into the servo format. The encoding may be any scheme in which there are first and second states representative of information; for example, the first interval may represent a digital one and the second interval may represent a digital zero, or vice versa. Thus, the information is encoded according to an amplitude modulation (AM) scheme.
In another embodiment, the frequencies of the signals used to create the first and second sections (i.e., the first interval) are the same, as are the frequencies of the third and fourth sections (the second interval), but they are not necessarily equal to each other. The system further comprises circuitry, responsive to the signal frequency, for decoding linear position information encoded into the servo format. The encoding may be any scheme in which there are first and second states representative of information; for example, the first interval may represent a digital one and the second interval represent a digital zero, or vice versa. Thus, the information is encoded according to a frequency modulation (FM scheme.
In the AM scheme, the frequencies of the signals written to create the respective sections may or may not be equal to each other. In the FM scheme, the mean amplitudes of the first and second intervals may or may not equal each other. Thus, it is possible (but not required) to combine the AM and FM schemes by varying both the amplitude and frequency of the signals.